Copyright laws and rules are wide, varied, vague, and can contradict from one government to another. So it's no wonder a lot of myths and perceptions of what you can and can not do continue to perpetuate. There are even legal cases where the judge ruled in favor of the wrong party because of confusion in how the law was interpreted.
For example, if I have a web site and insert an image into a web page and provide a link and credit to the person who owns the image, many people think it's OK to do that. It's not OK. You do not have the right to use the image unless you receive permission. "Permission" may be written notice or payment of a fee or both. Even if I state this, there will be people who will disagree because it is one of those many misconceptions.
A few things I have come across at DreamsTime:
(1) I was doing some research for image ideas and found that someone had uploaded a logo for a sporting event as if it were their own image. I caught it just after DT caught it and it has been removed from the database. It had been made available as an editorial but that doesn't give you the right to sell the image. "Editorial" is not a label that allows you to sell an image that does not belong to you.
(2) I have been finding instances with illustrations where they have been copied from other images that you can find on the internet. I won't say it's stealing because it's possible the illustrator created both versions of the image. But if that is not the case, you could be putting yourself and DT at risk for copyright infringement.
(3) Also, a while back someone posted a blog on how to get "ideas" for illustrations. The method was so search the internet for photographs and then trace them into illustrations. Even if you create the new version by hand, that does not give you the right to sell your image commercially. (The account in question was soon removed from DT soon after that blog was posted).
Perhaps it's confusing for many because there are some scenarios where it is legal. If I create an image of the logo for General Electric and sold the image as ART, it is OK to do that. There is a fine line here. Calling something "art" and the same image "stock" are two very different things. I can sell my painting of the logo and sell prints of the painting, too, but only as art. I am selling the image as my CREATION; I am NOT selling the USE of the LOGO. Huge difference! Even then, other rules apply to where this may not be OK and is too complicated for a short blog.
Some basic rules:
1. You can't photograph another photograph and sell it as your own
2. You can't copy another illustration and sell it as your own even if you do it in your own style
3. You can't make illustrations from photographs that do not belong to you
If it perfectly all right to use images of others to see how to draw the body or parts of the body, to get ideas, and to see how others presented a concept that you already have an idea for. In both worlds of art and stock, there really aren't any original ideas; it's all been done before. What hasn't been done before is your own personal style for an idea or finding new twists to old concepts. But... reworking someone else's image and then selling it as your own, people make many mistakes, and some of them have been very costly.
Since you have to post at least one image in a blog, I will leave with some examples where it is OK to have the same image that someone else did; but I hope you are starting to understand why. :-)